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\begin{equation}
	\Rightarrow P_1 = \pi \delta^2 n_A n_B \sqrt{\frac{8 k_B T}{\pi M}} \sim \underbrace{A}_{\mbox{Empirische Konstante / Frequenzfaktor [1/s]}} \overbrace{n_A n_B T^{1/2}}^{\mbox{Abhängigkeit Energie}}
\end{equation}

\documentclass{scrartcl}
\usepackage[utf8]{inputenc}

\usepackage{amsmath}

\usepackage{tikz}
\newcommand\glbeschr[6][]{%
  \text{\makebox[0pt][#5]{\tikz\draw[#1](0,0)|-(#2,#3)node[anchor=#4]{#6};}}%
  }

\usepackage{blindtext}

\begin{document}
\blindtext
\begin{gather} 
  \Rightarrow P_1 = \pi \delta^2 n_A n_B \sqrt{\frac{8 k_B T}{\pi M}} \sim 
    \underbrace{A}_{*} 
    \overbrace{ n_A n_B T^{1/2}}^{**}\\
  \makebox[\linewidth][l]{\footnotesize\begin{tabular}{rl}
     *&Empirische Konstante / Frequenzfaktor [1/s]\\
     **&Abhängigkeit Energie\\
    \end{tabular}}\notag
\end{gather}
\blindtext
\begin{equation} 
  \Rightarrow P_1 = \pi \delta^2 n_A n_B \sqrt{\frac{8 k_B T}{\pi M}} 
    \sim \underbrace{A}_{\glbeschr{0}{-0.5}{north}{c}{Empirische Konstante / Frequenzfaktor [1/s]}}
    \overbrace{ n_A n_B T^{1/2}}^{\glbeschr[<-,blue]{-0.5}{0.5}{east}{r}{Abhängigkeit Energie}} 
\end{equation}
\blindtext
\end{document}